1. Field of the Invention
The present invention relates to functional metal alloy particles. More particularly, the present invention is concerned with functional metal alloy particles containing substantially no lead, each exhibiting a plurality of different melting points including an original lowest melting point (a) and a highest melting point, wherein each of the metal alloy particles exhibits the original lowest melting point (a) at least at a surface portion thereof, and wherein, when each metal alloy particle is heated at a temperature equal to or higher than the original lowest melting point (a) to melt at least a surface portion of each metal alloy particle which exhibits the original lowest melting point (a), followed by cooling to room temperature to thereby solidify the melted portion of each metal alloy particle, the resultant solid metal alloy particle having experienced the melting and solidification exhibits an elevated lowest melting point (a′) higher than the original lowest melting point (a). The present invention is also concerned with a method for producing the metal alloy particles.
In the production of a conductive adhesive, an anisotropic conductive film, a soldering paste and the like, the metal alloy particles of the present invention can be advantageously used as a conductive filler which does not contain very poisonous lead used in conventional conductive fillers and, hence, exhibits high safety. Further, the metal alloy particles of the present invention have the following advantages. For example, when the conductive adhesive or soldering paste, each of which contains the metal alloy particles of the present invention as a conductive filler, is used for conductively connecting a semiconductor device or an electronic part to a substrate for an electronic circuit (hereinafter, a “substrate for an electronic circuit” is frequently referred to simply as a “substrate”), such as a printed circuit board (i.e., when the conductive adhesive or soldering paste is used for mounting the device or part on the substrate), the mounting is conducted by heat treatment to melt the conductive adhesive or soldering paste, which is deposited between the device or part and the substrate, followed by cooling to solidify the melted adhesive or paste. In such a case, by the use of the metal alloy particles of the present invention in the conductive adhesive or soldering paste, even when the above-mentioned heat treatment (hereinafter, referred to as the “initial heat treatment”) for mounting is conducted at a temperature which is lower than a heating temperature conventionally employed for the mounting, the device or part can be securely mounted on the substrate. In addition, after the initial heat treatment for mounting, the metal alloy particles contained in the conductive adhesive or soldering paste exhibit an elevated lowest melting point higher than its original lowest melting point, so that, even when the substrate having mounted thereon the device or part is subjected to further heat treatment (conducted for mounting another device or part on the substrate) at the same temperature as employed for the initial heat treatment, the metal alloy particles contained in the conductive adhesive or soldering paste are not melted and, hence, it is possible to prevent displacement of the semiconductor device or electronic part (that is, the conductive adhesive or soldering paste has excellent reliability with respect to heat resistance). Further, the conductive adhesive or soldering paste is advantageous in that, even when the conductive adhesive or soldering paste is exposed to high temperature conditions, the conductive adhesive or soldering paste can maintain the stand-off between the semiconductor device or electronic part and the substrate. The term “stand-off” means a state in which the conductive adhesive or conductive solid formed from the soldering paste, which electrically connects the device or part to the substrate, maintains a desired thickness, thereby maintaining a desired distance between the substrate and the conductive adhesive or conductive solid formed from the soldering paste. When the thickness of the conductive adhesive or conductive solid formed from the soldering paste becomes too small (i.e., when the stand-off is not maintained), various disadvantages (such as short-circuiting) may occur. In the case of the anisotropic conductive film comprising the metal alloy particles of the present invention, the anisotropic conductive film has the following advantages. When the anisotropic conductive film is used for production of an electronic part comprising an anisotropic conductive film having disposed on each surface thereof a plurality of electrodes, wherein the electrodes are attached to the anisotropic conductive film by heat treatment, the anisotropic conductive film can be securely attached to the electrodes at heating temperatures which are lower than a conventionally employed temperature. In addition, even when the electrodes are disposed on the anisotropic conductive film in a fine-pitch (closely spaced) arrangement, a high current density and a high conductivity can be achieved. Therefore, the electronic part obtained can be advantageously used, for example, for producing a color liquid crystal panel having a high density.
2. Prior Art
In recent years, for alleviating the defects of electric lead wiring, which defects are caused due to the developments of multi-pitch and fine-pitch technologies used in the multi-chip module (MCM) or the quad flat package (QFP), various connection methods for mounting a semiconductor device or electronic part on a substrate for an electronic circuit have been proposed. Examples of such methods include a ball grid array (BGA) connection method, a chip size packaging (CSP) connection method, and a flip chip (FC) connection method using a conductive adhesive, an anisotropic conductive film (ACF) or an anisotropic conductive paste (ACP).
In any of the conventional connection methods, a conventional Sn/Pb eutectic solder (comprising 63% by weight of Sn and 37% by weight of Pb) has been mainly used. In the conventional connection methods, generally, a semiconductor device or an electronic part is conductively connected to a substrate (i.e., the device or part is mounted on the substrate) by a method in which an Sn/Pb eutectic solder is disposed between the device or part and the substrate, followed by heat treatment (to melt the solder) and subsequent cooling treatment (to solidify the melted solder) in a reflow furnace or the like (hereinafter, the heat treatment for mounting a semiconductor device or an electronic part to a substrate is frequently referred to simply as the “heat treatment for mounting”).
With respect to an Sn/Pb eutectic solder ball, the solder ball itself is capable of forming an conductive connection. By virtue of this property, the Sn/Pb eutectic solder ball is used in the BGA connection method, the CSP connection method and the FC connection method.
Examples of manners for using the Sn/Pb eutectic solder ball in the BGA connection method, the CSP connection method or the FC connection method include: a manner in which the Sn/Pb eutectic solder ball is placed between a semiconductor device-containing electronic package and a substrate, followed by soldering the package on the substrate; and a manner in which a semiconductor device is directly soldered on an interposer (i.e., a substrate for an electronic package) using the Sn/Pb eutectic solder ball.
The Sn/Pb eutectic solder is a binary eutectic alloy and has a melting point of 183° C. (at which temperature, in the phase diagram of the Sn/Pb eutectic solder, the solidus curve intersects with the liquidus curve). Particles of the Sn/Pb eutectic solder are uniformly melted at temperatures which are higher than 183° C. Therefore, the use of the Sn/Pb eutectic solder ball in the above-mentioned connection methods is advantageous in that a semiconductor device and a substrate can be securely attached to each other at a relatively low temperature.
However, the Sn/Pb eutectic solder which has been used for soldering is re-melted at a temperature higher than the melting point thereof (i.e., 183° C.), so that Sn/Pb eutectic solder has a poor reliability with respect to heat resistance. More specifically, for example, in the case of a substrate on which a first electronic part is soldered using the Sn/Pb eutectic solder, when it is attempted to mount a second electronic part on the substrate by heat treatment, it is highly possible that the Sn/Pb eutectic solder used for soldering the first electronic part is re-melted, thereby causing displacement of the first electronic part.
In an attempt to prevent such re-melting and displacement of an electronic part, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-6073, Unexamined Japanese Patent Application Laid-Open Specification No. 2000-210767 (corresponding to EP 1 002 612) and the like propose a method utilizing the metal diffusion phenomenon induced by heat. Specifically, in the method of these patent documents, an Sn/Au alloy is used for mounting electronic parts on a substrate (each of Sn and Au is relatively easy to diffuse in the alloy by heat treatment conducted for mounting), wherein the composition of the Sn/Au alloy is caused to change by the thermal diffusion of the metals during the heat treatment for mounting the electronic part on the substrate. In this method, it is attempted to elevate the melting point of the Sn/Au alloy by changing the composition thereof, so as to prevent the Sn/Au alloy from re-melting during the subsequent heat treatment for mounting another electronic part on the substrate. However, by this method, it is difficult to stably obtain a metal alloy having a desired composition after the thermal diffusion of the metals. If a metal alloy having a desired composition is stably obtained, this means that an intermetallic compound, which has a stable composition and a melting point higher than those of Sn and Au, is formed in the alloys after the thermal diffusion of the metals (Sn and Au), that is, it becomes possible to form a high melting point portion (composed of the above-mentioned intermetallic compound) in the alloy, which portion is not melted at the temperature generally employed for the above-mentioned heat treatment for mounting. However, on the other hand, the lowest melting point of the alloys remains unchanged even after the thermal diffusion of the metals. Therefore, when the substrate having mounted thereon the electronic parts is heated again at the temperature employed for the heat treatment for mounting, the alloy is re-melted at least at a portion thereof. Accordingly, the metal alloy has a poor reliability with respect to heat resistance.
Further, the above-mentioned method has the following disadvantage. In the method, the above-mentioned Sn/Au alloy is formed by interposing a laminate of a plurality of metal layers (including an Sn layer and an Au layer) between the electronic part and the substrate, followed by heating. After the formation of the Sn/Au alloy, the heating is continued, while strictly controlling the heating conditions, so as to adjust the composition of the Sn/Au alloy. Therefore, cumbersome operations are needed for practicing the method.
As seen from the above, there has not yet been known a conductive adhesive material having reliability with respect to heat resistance, which can be put into a practical use, and it has been desired to develop such a conductive adhesive material.
The above-mentioned Sn/Pb eutectic solder ball also has the following defect. When the Sn/Pb eutectic solder ball is caused to bear a load during the operation of attaching a semiconductor device to a substrate, the Sn/Pb eutectic solder ball is crushed. As a result, the Sn/Pb eutectic solder ball cannot maintain the stand-off between the semiconductor device and the substrate, and, in addition, the mutually adjacent Sn/Pb eutectic solder balls are bonded together and unified. The reason for this unification is that the Sn/Pb eutectic solder ball is uniformly melted by heating.
For overcoming the defects of Sn/Pb eutectic solder, it is attempted to obtain an Sn/Pb non-eutectic solder having a composition wherein the melting point of the Sn/Pb non-eutectic solder becomes higher than that of the Sn/Pb eutectic solder. However, this non-eutectic Sn/Pb solder has the following disadvantage. When the Sn/Pb non-eutectic solder is used, the temperature for soldering necessarily becomes high, as compared to that employed in the soldering using the Sn/Pb eutectic solder, so that it is likely that both of the semiconductor device and the substrate suffer undesired thermal influences, leading to a deterioration of the semiconductor device and/or the substrate. For avoiding such disadvantage, it is desired to use an Sn/Pb solder which can be used at a temperature of 250° C. or less.
Thus, it has been desired to develop a conductive adhesive material satisfying the following requirements:    (1) the conductive adhesive material should have heat resistance reliability, so that, even when the conductive adhesive material (which has been used for mounting an electronic part or the like on a substrate) is repeatedly subjected to heat treatment for mounting electronic parts and the like, the conductive adhesive material is capable of preventing displacement of the electronic part which has already been mounted on the substrate;    (2) the conductive adhesive material should have connection stability, that is, the conductive adhesive material should be capable of maintaining the stand-off between the electronic part and the substrate; and    (3) the conductive adhesive material should be capable of strongly attaching an electronic part or the like to a substrate even by a heat treatment at a relatively low temperature (i.e., about 250° C. or less) which does not adversely affect the electronic part and the substrate.
As apparent from the above explanation of the prior art, a conductive adhesive material satisfying the requirement (1) above has not yet been developed. With respect to the requirements (2) and (3) above, a number of proposals have been made to alleviate the defects of the Sn/Pb eutectic solder and to develop a conductive adhesive material satisfying the requirement (2) or (3).
As an example of methods proposed to alleviate the above defects, there can be mentioned a method in which, for maintaining the stand-off between the semiconductor device and the substrate, very small balls of a high melting point metal, such as Au, Ag or Cu, are used instead of Sn/Pb eutectic solder balls. More specifically, in this method, the very small balls of a high melting point metal are interposed between the semiconductor device and the substrate, and the balls are bonded to the semiconductor device and the substrate using an Sn/Pb eutectic solder. However, in this method, it is necessary to prevent oxidation of the surfaces of the high melting point metal balls by, for example, plating or precoating the Sn/Pb eutectic solder, leading to a disadvantage that the mechanical strength of the solder connection between the semiconductor device and the substrate inevitably becomes poor. As a result, the semiconductor device is easily disconnected from the substrate even by a small load caused by a small impact, vibration or the like. Further, the high melting point metal is expensive and, hence, the use of the high melting point metal is disadvantageous also from an economical point of view.
In an attempt to maintain the stand-off between the semiconductor device and the substrate even when the heat treatment for mounting is repeatedly conducted by the use of a reflow furnace, a method is proposed which uses an Sn/Pb eutectic soldering paste having incorporated therein metal wires or metal particles (which would not get melt-mixed with the Sn/Pb eutectic solder even when the heat treatment for mounting is repeatedly conducted by the use of a reflow furnace) (see, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 2-134897 and Hei 7-171693). However, in this method, it is difficult to disperse metal wires or metal particles uniformly in the Sn/Pb eutectic soldering paste and, hence, it is difficult to maintain the stand-off between the semiconductor device and the substrate.
As another example of methods proposed to maintain the stand-off between the semiconductor device and the substrate even when the heat treatment for mounting is repeatedly conducted by the use of a reflow furnace, there can be mentioned a method which uses an Sn/Pb eutectic soldering paste containing, as a conductive filler, particles of metals (as simple substances) or a metal alloy, and in which a metal alloy is formed in the Sn/Pb eutectic soldering paste during the heat treatment of the paste in the reflow furnace by utilizing the difference in the ionization tendency between the metals in the Sn/Pb eutectic soldering paste. For example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 9-295182 uses Sn, Ag, Bi, In, Cu and Zn as the metals (as simple substances) and obtains an Sn/Ag alloy, an Sn/Cu alloy, an Sn/Bi alloy, an Sn/Zn alloy and Sn/In alloy.
Further, a method is known which uses an Sn/Pb eutectic soldering paste containing a metal alloy powder as a conductive filler, which metal alloy powder exhibits a plurality of melting points before the heat treatment thereof in the reflow furnace and exhibits a single melting point after the heat treatment thereof in the reflow furnace.
However, these methods are disadvantageous not only in that it is difficult to disperse the filler uniformly in the Sn/Pb eutectic soldering paste, but also in that the metal alloy obtained does not have a uniform composition due to the difference in specific gravity between the metals or alloys contained in the soldering paste. Further, the above Sn/Pb eutectic soldering paste after melting by the heat treatment thereof in the reflow furnace exhibits disadvantageously poor uniformity in composition, and disadvantageously poor reproducibility of composition. Therefore, these methods are not practicable.
Generally, in a phase diagram of a metal alloy, the metal alloy composition corresponding to a point at which the solidius curve intersects with the liquidus curve is a eutectic composition. A metal alloy having a eutectic composition exhibits a single melting point having substantially no temperature range. Therefore, such a metal alloy is uniformly melted at the melting point thereof. For example, as mentioned above, the Sn/Pb eutectic solder is uniformly melted at the melting point thereof (i.e., 183° C.). On the other hand, in the case of a metal alloy which does not have a eutectic composition (i.e., metal alloy having a composition corresponding to a point at which the solidius curve does not intersect with the liquidus curve), the metal alloy has either a single melting point having a temperature range (i.e., there is a difference between the lowest temperature at which a part of the metal alloy starts to melt and the lowest temperature at which the metal alloy finishes melting) or a plurality of melting points.
As examples of patent documents in which the use of a metal alloy having a plurality of melting points is described, there can be mentioned Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 9-174278 and Hei 9-206983. In these patent documents, it is attempted to lower a melting point of a solder containing no lead. However, the highest melting point of the solder containing no lead is lower than the temperature (230 to 250° C.) which is generally employed for mounting an electronic part on a substrate. Therefore, the solder cannot maintain the stand-off during the mounting of an electronic part on the substrate.
For the purpose of improving the connecting strength of a via hole conductor (which connects electronic parts to each other through a via hole), Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-214575 describes a method for producing a circuit board, which comprises: forming a via hole in an insulating layer; filling the via hole with a conductive paste (containing a high melting point filler) to form a via hole conductor having its both end portions exposed at both end openings of the via hole; applying a paste containing a low melting point alloy (such as an Sn/In alloy) on each of the end portions of the via hole conductor in a predetermined thickness; and placing a metal circuit layer on both sides of the insulating layer, followed by heating, to cause the low melting point alloy to form intermetallic compounds with the high melting point conductive filler (in the via hole conductor) and the metal circuit layer. However, the low melting point alloy has an invariable specific melting point, so that, when the obtained circuit board is re-heated at the same temperature for the formation of the intermetallic compounds, the low melting point alloy is inevitably melted. That is, the via hole conductor has a poor reliability with respect to heat resistance.
As apparent from the above, a conductive adhesive material satisfying the requirements of (1) to (3) above has not yet been developed by conventional methods.
Further, it should be noted that a solder containing lead, such as an Sn/Pb eutectic solder, has the following serious defects. Lead is very poisonous and, hence, a lead-containing solder is harmful to humans. Further, lead radiates α-rays, so that, when a lead-containing solder is placed near the semiconductor device, the solder may cause the malfunction of the semiconductor device. Therefore, the mounting of the semiconductor device must be conducted in a manner such that the lead-containing solder is kept away from the semiconductor device.
Thus, it has been desired to develop a conductive adhesive material satisfying the requirements of (1) to (3) above, which contains substantially no lead and which is free of the defects of the Sn/Pb eutectic solder.